void operator

This operator allows evaluating expressions that produce a value into places where an expression that evaluates to undefined is desired.

The void operator is often used merely to obtain the undefined primitive value, usually using void(0) (which is equivalent to void 0). In these cases, the global variable undefined can be used.

It should be noted that the precedence of the void operator should be taken into account and that parentheses can help clarify the resolution of the expression following the void operator:

void 2 === "2"; // (void 2) === '2', returns false
void (2 === "2"); // void (2 === '2'), returns undefined

When using an immediately-invoked function expression, the function keyword cannot be at the immediate start of the statement, because that would be parsed as a function declaration, and would generate a syntax error when the parentheses representing invocation is reached — if the function is unnamed, it would immediately be a syntax error if the function is parsed as a declaration.

function iife() {
  console.log("Executed!");
}(); // SyntaxError: Unexpected token ')'

function () {
  console.log("Executed!");
}(); // SyntaxError: Function statements require a function name

In order for the function to be parsed as an expression, the function keyword has to appear at a position that only accepts expressions, not statements. This can be achieved by prefixing the keyword with a unary operator, which only accepts expressions as operands. Function invocation has higher precedence than unary operators, so it will be executed first. Its return value (which is almost always undefined) will be passed to the unary operator and then immediately discarded.

Of all the unary operators, void offers the best semantic, because it clearly signals that the return value of the function invocation should be discarded.

void function () {
  console.log("Executed!");
}();

// Logs "Executed!"

This is a bit longer than wrapping the function expression in parentheses, which has the same effect of forcing the function keyword to be parsed as the start of an expression instead of a statement.

(function () {
  console.log("Executed!");
})();

Note that this trick only applies to IIFEs defined with the function keyword. Attempting to use the void operator to avoid parentheses for an arrow function results in a syntax error. Arrow function expressions always require parentheses around them when being called.

void () => { console.log("iife!"); }(); // SyntaxError: Malformed arrow function parameter list

When a browser follows a javascript: URI, it evaluates the code in the URI and then replaces the contents of the page with the returned value, unless the returned value is undefined. The void operator can be used to return undefined. For example:

<a href="javascript:void(0);">Click here to do nothing</a>

<a href="javascript:void(document.body.style.backgroundColor='green');">
  Click here for green background
</a>

Arrow functions introduce a short-hand braceless syntax that returns an expression. This can cause unintended side effects if the expression is a function call where the returned value changes from undefined to some other value.

For example, if doSomething() returns false in the code below, the checkbox will no longer be marked as checked or unchecked when the checkbox is clicked (returning false from the handler disables the default action).

checkbox.onclick = () => doSomething();

This is unlikely to be desired behaviour! To be safe, when the return value of a function is not intended to be used, it can be passed to the void operator to ensure that (for example) changing APIs do not cause arrow functions' behaviors to change.

checkbox.onclick = () => void doSomething();

• undefined